This invention relates to an exhaust gas purifying device for purifying hydrocarbons (HC) and carbon monoxide (CO) contained in exhaust gases from an internal combustion engine in the course of exhaust through an exhaust system, and more particularly to a secondary air flow rate control device for feeding the purifying air (which will be referred to as "the secondary air", hereinafter) to the exhaust gas purifying device of the engine. Hitherto, various kinds of purifying devices such as a catalyst convertor or an afterburner device have been incorporated in the exhaust system of an internal combustion engine for reducing the amounts of HC and CO contained in exhaust gases from an engine of a motor vehicle. Those devices dictate the use of the secondary air for purifying or oxidizing hydrocarbons or carbon monoxide contained in exhaust gases. In this respect, the secondary air is supplied from an air pump driven by means of an internal combustion engine or a motor.
However, those devices suffer from disadvantages in that, because the capacity of the aforesaid air pump is set so as to feed the maximum amount of the secondary air particularly when such is required during the running of an internal combustion engine, the amount of the secondary air being fed to the engine in the other running conditions, i.e., in the low running or decelerating condition becomes excessive in amounts, so that the cooling effect of the secondary air predominates over its reacting effect for the exhaust gas purifying device. To overcome this shortcoming, there has been proposed an attempt, in which a valve is provided in an air passage leading the secondary air from an air pump to the exhaust gas purifying device, and thus the aforesaid valve is drivine due to the suction negative pressure in carburetor or the like which supplies a mixture charge to the internal combustion engine, for the purpose of varying the cross sectional area of the aforesaid air passage, thus controlling the amount of the secondary air to be fed thereto. However, such an attempt to limit or throttle the cross sectional area of the air passage is tantamount to an attempt to limit the area of a delivery port of the air pump, so that it is not recommendable from the viewpoint of the durability of an air pump. Recently, there has been proposed another attempt, in which part of an air passage is open to atmosphere by providing an aperture and a valve is provided in the aforesaid aperture, whereby the secondary air to be supplied to the engine at the time of low load or decelerating condition thereof is bled into the atmosphere, thereby reducing the amount of the secondary air being fed to the exhaust gas purifying device in an attempt to promote or accelerate the reaction taking place in the exhaust gas purifying device. For better understanding of the background of the present invention, it may be advantageous to give more detailed description of this conventional device. The valve provided in the aforesaid aperture open to atmosphere is secured to a servo-diaphragm adapted to be driven due to the negative pressure in a suction pipe, and then the servo-diaphragm is in communication with the downstream of a carburetor via a negative pressure passage. In addition, an electromagnetic valve is built in this negative pressure passage, being connected to a micro-switch in an acceleration sensor. Since at the time of deceleration or low loading condition of an engine, the micro-switch remains inoperable and the electromagnetic valve keeps the negative pressure passage open, the negative pressure is applied to the servo-diaphragm and the valve is open so as to bleed the air fed from the air pump into atmosphere. With such an arrangement, when the engine is accelerated, the micro-switch is so actuated as to energize the electromagnetic valve to block the negative pressure which is being applied to the servo-diaphragm, while the valve closes the aperture open to atmosphere, thereby feeding a great amount of the secondary air to the exhaust gas purifying device. Thus, the actuation of the accelerating pump permits the purification or oxidation of HC and Co of an increased amount.
However, with the secondary air flow rate control device for use in an exhaust gas purifying device of such an arrangement, there have been encountered disadvantages in that, since the negative pressure is applied to the diaphragm of the acceleration sensor and in addition the air flow rate control portion is driven by feeding an electric current to the electromagnetic valve by way of the micro-switch or since the length of the negative pressure passage becomes excessive due to the layout of an engine room, there results delayed responsive characteristic of the air flow rate control portion to the variation in the negative pressure. This entails limitations on the purifying characteristics of the exhaust gas purifying device. On the other hand, in the light of the facts that various parts for use in the exhaust gas purifying device as well as safety means are to be built in an engine room in the near future, it is not reccommendable from the viewpoints of the layout of an engine room to individually prepare the parts for the acceleration sensor, electromagnetic valve, air flow rate control portion and the like for mounting same within the engine room. This further poses another disadvantage such as difficulties in handling and maintenance, and increased cost of manufacture. In addition, because of the use of the micro-switch, there necessarily be incurred frequent troubles in the micro-switches due to vibration in motor vehicle, thus causing the failure to operate the air flow rate control portion in the normal condition.